This invention relates generally to logging while drilling wherein measurements of downhole conditions within a borehole are telemetered to the surface of the earth by means of a downhole acoustic transmitter which produces a continuous acoustic wave that passes upward through the drilling fluid. More particularly, it relates to a DC/AC motor drive that converts a DC power source to an AC power source for operating a two-phase AC induction motor in the downhole acoustic transmitter.
In the past, a conventional practice in the logging of a borehole has been to apply electric current from a suitable source aboveground through an insulated conductor extending into the borehole to sensing apparatus. The sensing apparatus provides a signal in the insulated conductor representative of the characteristic measured within the borehole. The provision and maintenance of such an insulated conductor for logging the borehole while simultaneously drilling the borehole has been found to be impractical.
More recently, logging-while-drilling systems have been employed which do not require an insulated conductor in the borehole at any time for logging operations. In one such system, the sensing apparatus located within the borehole transmits the logging measurements by means of an acoustic wave passing upward through the drill string. An example of such a system is disclosed in U.S. Pat. No. 2,810,546 to B. G. Eaton et al. In another such system the drilling liquid within the borehole is utilized as the transmission medium for the information-bearing acoustic waves. An example of such a system is disclosed in U.S. Pat. No. 3,309,656 to John K. Godbey. In the Godbey systems, drilling fluid is continuously circulated downward through the drill string and drill bit and upward through the annulus provided by the drill string and the borehole wall, primarily for the purpose of removing cuttings from the borehole. An acoustic transmitter located downhole continuously interrupts the flow of the drilling fluid, thereby generating an acoustic wave in the drilling fluid. The acoustic wave is modulated with information measured downhole by sensing apparatus, and the modulated acoustic wave is telemetered uphole through the drilling fluid to suitable recording equipment.